1. Field of the Invention
The invention generally relates to a device and a method for optically testing a surface.
2. Description of the Related Art
An image-recording device for imaging flaws on a planar, reflective surface, where a telecentric optical train is employed for imaging that zone of the surface to be investigated using a camera is described in U.S. Pat. No. 6,870,949B2. The '949 patent describes the dark-field illumination of the surface as being provided by an annular illumination source arranged coaxial to the camera's optical axis. That device also provides that light from the light sources for illuminating surfaces is reflected normal to the camera's optical axis by a semitransparent mirror, in which case, facilities for implementing bright-field illumination are also provided. In the latter case, the illuminated spot on the surface under investigation is imaged onto the camera lens by the field lens and the beam path transits the semitransparent-mirror's plane-parallel plate, thereby causing astigmatism that reduces image fidelity, particularly in the case of large beam divergences, such as whenever field lenses having short focal lengths are employed.
A beam splitter for an illumination-side and sensor-side, telecentric, optical train that provides for at least a transparent masking of the semitransparent mirror in order to prevent stray-light effects due to the presence of contamination is described in U.S. Pat. No. 4,561,722A.
A manuscript reader having a camera and nearly on-axis, dark-field illumination, where the same telecentric lens is used for illumination and the camera's optical train, is described in Japanese patent application JP 08172506A. Its nearly on-axis, dark-field illumination yields improved character recognition, particularly on glossy paper.
The arrangements having a camera whose lens lies on the axis of the telecentric optical train that are described in U.S. Pat. No. 6,870,949B2 and JP 08172506A have the disadvantage that the heights of the entire arrangements, as measured along their field lens' optical axis, exceed their field lens' focal length by more than the overall length of their camera and camera lens.
Common to all arrangements that employ field lenses is that reflections that may be optimally suppressed only over a narrow wavelength range by coating their surfaces occur at the front and rear surfaces thereof. Chromatic aberration that can be only partially avoided by employing expensive, achromatic, lens systems occurs, particularly in cases where polychromatic light is used for investigating surfaces. Furthermore, more often than not, all arrangements that employ telecentric optical trains have undesirably large heights, as measured normal to the surfaces under investigation, which can present problems, particularly in cases where space in the vicinities of those surfaces is at a premium. Even employing a planar, beam-deflecting mirror between those surfaces and the field lens may frequently be insufficient to allow adequately reducing the dimensions of overall arrangements, as measured normal to the surfaces under investigation.
Preferred embodiments of the invention shall be described in detail below, based on the accompanying figures.